The invention described herein is in general an intelligent ultrasonic measuring apparatus. More specifically, in this invention is directed to a means of measuring the average tensile stress within a bolt or tensile load member when the fastener is tightened against the structure which it fastens.
The accurate measurement of tensile stress in threaded fasteners is essential to control of the clamping force exerted by such fasteners, and the prevention of failure of the fastener by determination of the tensile stress relative to the yield stress of the fastener material.
Throughout the history of threaded fasteners, a number of devices have been developed and employed with the purpose of measuring and/or controlling such tensile stress.
Torque wrenches attempt to control such stress by determining the rotational energy applied to the nut, such measurement is highly inaccurate due to the variable friction forces between nut and bolt and nut and structure or washer.
Hydraulic tensioners apply accurate tensile load to the fastener. However, relaxation of this load upon the nut make accurate control extremely difficult.
Mechanical micrometers have been employed to measure the elongation of a fastener upon tightening which occurs in proportion to the applied tensile stress. Such devices are extremely time-consuming to operate and subject to operator error.
Various ultrasonic (Echo ranging) extensometers ranging from standard thickness gauges which measure subsequent lengths of the fastener to units which measure elongation directly are generally limited in the overall length of the fastener or the elongation which can be measured. These units measure the apparent change in length of the fastener thereby making no adjustment for the differing sonic phenomena occurring in the stressed and unstressed portion of the fastener, nor are these units readily adaptable to various fastener materials or change in fastener temperature. Thus, accurate correlation of apparent change in fastener length indicated by these units is not practical.
It is, therefore, apparent that the devices described above do not provide a practical means of measuring or controlling compressive or tensile stress in bolts or similar structural members. It is for this specific purpose that we have developed our invention.
Typical of these ultrasonic extensmeters is McFaul, et al., U.S. Pat. No. 3,759,090 in which an analog device for measuring the elongation of a bolt is disclosed. In McFaul, an ultrasonic pulse was inserted into the bolt to be measured, and the echo pulse was received and processed. The instant of receipt of the echo pulse for a non-elongated bolt was adjusted to occur just subsequent to a periodically occurring marker pulse, the time period between each marker pulse occurrence corresponding to an indicatable elongation of the bolt. The time period between the incidence of the echo pulse and the occurrence of the next marker pulse was converted into an analog meter reading, the analog meter needle deflection being calibrated to indicate units of elongation. As the bolt was elongated, the time for the applied pulse to travel down the length of the bolt and to be reflected back being increased. This resulted in the time period between the incidence of the echo pulse and the occurrence of the next reference pulse being shortened. As a result, the meter indication also decreased. In this manner, as the bolt was elongated, the user could monitor the change in elongation by watching the meter reading decrease.
While the ultrasonic extensometer of McFaul permitted the user to monitor an indication of elongation, the actual stress to which the bolt was being subjected was unknown. Additionally, because the meter indicated the time period between each receipt of the echo pulse and the next occurring reference pulse, the movement of the meter was susceptible to the usual jitter which results when a series of individual samples are each displayed. Additionally, because of variations in echo pulse amplitude from sample to sample, the exact time of receipt of the echo pulse was not constant from sample to sample. This further added to the jitter in the meter display, as well as to a degradation in the accuracy of the indicated elongations. Because the ultrasonic extensometer of McFaul require that the user initially "zero-out" the non-elongated portion of the bolt by setting the received echo pulse to occur just subsequent to the reference pulse, the elongation reading which resulted was further compromised by any "zeroing" error which occurred. Additionally, the analog circuits used to process the ultrasonic signals were susceptible to both short term and long term drift causing excessively long warm-up times and unreliable repeatability of readings.